Pressure limiting oil-hydraulic valves which can be inserted in a hydraulic circuit in order to separate (or sealingly isolate) a high pressure portion (or zone) of the hydraulic circuit from a low pressure portion (or zone) of the hydraulic circuit are already known, e.g. from patent documents per WO2006/069692, US2006/039811 and US2005/264100.
These valves are inserted inside a receiving seat.
These valves comprise a valve body and a closing body (or closing member) which are arranged inside the receiving seat.
In particular the closing member is slidable with respect to the valve body along a first direction of longitudinal—or axial—extension, between a closed position in which the closing member bears against an associated seat of the valve body and an open position in which the closing body does not bear against the aforementioned seat and a fluid through-flow region is defined between stem and valve body.
The valve body has, formed therein, a central hole which defines the fluid through-flow region; the said valve body may have a guide for the stem of the closing member, which allows axial displacements of the latter.
The closing member comprises a substantially conical or spherical portion—or head—mating with the valve body and a stem with an elongated form extending from this head.
The stem of the closing body is provided with a threaded portion which is remote from the head.
The valve further comprises a sleeve for centring/guiding and locking the stem, which is slidable with respect to the walls of the seat receiving the said valve.
The centring/locking sleeve is provided with a threaded hole inside which the stem is screwed and locked.
This valve also comprises a spring extending longitudinally or axially along the first direction.
It should be noted that the spring is arranged between the sleeve and the valve body.
This valve normally isolates a high pressure region of the oil-hydraulic circuit from a low pressure region (closing member in closed position) and is designed to connect the high pressure region to the low pressure region (closing member in open position) if the relative pressure between the high pressure duct and the low pressure duct is greater than a predetermined operating pressure value of the valve (usually corresponding to an overpressure or shock condition in the high pressure circuit).
It should be noted that the prestress of the spring (compression of the spring when there is no pressure difference between high pressure region and low pressure region) determines the value of the operating pressure (value of the pressure difference between high pressure region and low pressure region).
Therefore, in this valve, correct adjustment of the prestress of the spring (so-called valve calibration) is of fundamental importance in order for the closing body to be moved from the closed position into the open position when there is a predefined pressure difference between high pressure region and low pressure region, corresponding to the desired operating pressure.
It should be noted that this prestress coincides with the resilient force developed by the spring when compressed to the first working length, usually predefined by the surrounding conditions for each valve size. According to the prior art, valve calibration is performed in situ, namely by connecting the valve to a “test” circuit between a high pressure zone and a low pressure zone.
This calibration method envisages screwing the sleeve onto the stem in a predefined position, inserting it inside the seat and applying a predetermined relative pressure between the high pressure circuit and low pressure circuit corresponding to the desired operating pressure value.
This calibration method is repeated until the closing member establishes communication between the two—high pressure and low pressure—zones at the set (required) pressure difference value between the two—high pressure and low pressure—regions.
Therefore, hitherto, with this calibration method, ensuring a high degree of precision for the valve calibration is particularly time-consuming.
In addition, a valve thus designed is relatively complex and requires long assembly times.
A further drawback encountered in this type of valve is that it does not guarantee a high degree of reliability or a high operating stability; in fact, this valve is unable to ensure over time that opening of the valve occurs at the operating value for which the valve has been calibrated (using the method described above).
This problem is partly solved by adopting additional locking systems for preventing unscrewing of the threaded pair, although this further complicates the structure and assembly of the valve.
This drawback is particularly critical in the applications where the opening precision is an essential requirement.